docs: def p_expression_binop(t):
      '''expression : expression PLUS expression
                    | expression MINUS expression
                    | expression TIMES expression
                    | expression DIVIDE expression'''
      if t[2] == '+'  : t[0] = t[1] + t[3]
      elif t[2] == '-': t[0] = t[1] - t[3]
      elif t[2] == '*': t[0] = t[1] * t[3]
      elif t[2] == '/': t[0] = t[1] / t[3]